39,498 research outputs found
Ab initio two-dimensional multiband low-energy models of EtMe_3Sb[Pd(dmit)_2]_2 and \kappa-(BEDT-TTF)_2Cu(NCS)_2 with comparisons to single-band models
We present ab initio two-dimensional extended Hubbard-type multiband models
for EtMe_3Sb[Pd(dmit)_2]_2 and \kappa-(BEDT-TTF)_2Cu(NCS)_2, after a
downfolding scheme based on the constrained random phase approximation (cRPA)
and maximally-localized Wannier orbitals, together with the dimensional
downfolding. In the Pd(dmit)_2 salt, the antibonding state of the highest
occupied molecular orbital (HOMO) and the bonding/antibonding states of the
lowest unoccupied molecular orbital (LUMO) are considered as the orbital
degrees of freedom, while, in the \kappa-BEDT-TTF salt, the
HOMO-antibonding/bonding states are considered. Accordingly, a three-band model
for the Pd(dmit)_2 salt and a two-band model for the \kappa-(BEDT-TTF) salt are
derived. We derive single band models for the HOMO-antibonding state for both
of the compounds as well.Comment: 10 pages, 9 figures, 3 tables; submitted to Physical Review
Localization length of a soliton from a non-magnetic impurity in a general double-spin-chain model
A localization length of a free-spin soliton from a non-magnetic impurity is
deduced in a general double-spin-chain model ( model). We have
solved a variational problem which employs the nearest-neighbor singlet-dimer
basis. The wave function of a soliton is expressed by the Airy function, and
the localization length is found to obey a power law of the
dimerization with an exponent -1/3; .
This explains why NaV_2O_5 does not show the antiferromagnetic order, while
CuGeO_3 does by impurity doping. When the gap exists by the bond-dimerization,
a soliton is localized and no order is expected. Contrary, there is a
possibility of the order when the gap is mainly due to frustration.Comment: 4 pages, REVTeX, Figures are in eps-file
Staggered Ladder Spectra
We exactly solve a Fokker-Planck equation by determining its eigenvalues and
eigenfunctions: we construct nonlinear second-order differential operators
which act as raising and lowering operators, generating ladder spectra for the
odd and even parity states. These are staggered: the odd-even separation
differs from even-odd. The Fokker-Planck equation describes, in the limit of
weak damping, a generalised Ornstein-Uhlenbeck process where the random force
depends upon position as well as time. Our exact solution exhibits anomalous
diffusion at short times and a stationary non-Maxwellian momentum distribution.Comment: 4 pages, 2 figure
Decoherence due to discrete noise in Josephson qubits
We study decoherence produced by a discrete environment on a charge Josephson
qubit by introducing a model of an environment of bistable fluctuators. In
particular we address the effect of noise where memory effects play an
important role. We perform a detailed investigation of various computation
procedures (single shot measurements, repeated measurements) and discuss the
problem of the information needed to characterize the effect of the
environment. Although in general information beyond the power spectrum is
needed, in many situations this results in the knowledge of only one more
microscopic parameter of the environment. This allows to determine which
degrees of freedom of the environment are effective sources of decoherence in
each different physical situation considered.Comment: 20 pages, 11 figure
Solid motor diagnostic instrumentation
A review of typical surveillance and monitoring practices followed during the flight phases of representative solid-propellant upper stages and apogee motors was conducted to evaluate the need for improved flight diagnostic instrumentation on future spacecraft. The capabilities of the flight instrumentation package were limited to the detection of whether or not the solid motor was the cause of failure and to the identification of probable primary failure modes. Conceptual designs of self-contained flight instrumentation packages capable of meeting these reqirements were generated and their performance, typical cost, and unit characteristics determined. Comparisons of a continuous real time and a thresholded hybrid design were made on the basis of performance, mass, power, cost, and expected life. The results of this analysis substantiated the feasibility of a self-contained independent flight instrumentation module as well as the existence of performance margins by which to exploit growth option applications
Finite spin-glass transition of the XY model in three dimensions
A three-dimensional XY spin-glass model is investigated by a
nonequilibrium relaxation method. We have introduced a new criterion for the
finite-time scaling analysis. A transition temperature is obtained by a
crossing point of obtained data. The scaling analysis on the relaxation
functions of the spin-glass susceptibility and the chiral-glass susceptibility
shows that both transitions occur simultaneously. The result is checked by
relaxation functions of the Binder parameters and the glass correlation lengths
of the spin and the chirality. Every result is consistent if we consider that
the transition is driven by the spin degrees of freedom.Comment: 11 pages, 8 figures, incorrect arguments are delete
Global Structure of Optically Thin, Magnetically Supported, Two-Temperature, Black Hole Accretion Disks
We present global solutions of optically thin, two-temperature black hole
accretion disks incorporating magnetic fields. We assume that the
{\pi}{\phi}-component of the Maxwell stress is proportional to the total
pressure, and prescribe the radial dependence of the magnetic flux advection
rate in order to complete the set of basic equations. We obtained magnetically
supported (low-{\beta}) disk solutions, whose luminosity exceeds the maximum
luminosity for an advection-dominated accretion flow (ADAF), L > 0.4 {\alpha}^2
L_Edd, where L_Edd is the Eddington luminosity. The accretion flow is composed
of the outer ADAF, a luminous hot accretion flow (LHAF) inside the transition
layer from the outer ADAF to the low-{\beta} disk, the low-{\beta} disk, and
the inner ADAF. The low-{\beta} disk region becomes wider as the mass-accretion
rate increases further. In the low-{\beta} disk, the magnetic heating balances
the radiative cooling, and the electron temperature decreases from ~ 10^9.5 K
to ~ 10^8 K as the luminosity increases. These results are consistent with the
anti-correlation between the energy cutoff in X-ray spectra (hence the electron
temperature) and the luminosity when L > 0.1 L_Edd, observed in the bright/hard
state during the bright hard-to-soft transitions of transient outbursts in
galactic black hole candidates.Comment: 27 pages, 15 figures, accepted for Publications of Astronomical
Society of Japa
M87 black hole mass and spin estimate through the position of the jet boundary shape break
We propose a new method of estimating a mass of a super massive black hole
residing in the center of an active galaxy. The active galaxy M87 offers a
convenient test case for the method due to the existence of a large amount of
observational data on the jet and ambient environment properties in the central
area of the object. We suggest that the observed transition of a jet boundary
shape from a parabolic to a conical form is associated with the flow transiting
from the magnetically dominated regime to the energy equipartition between
plasma bulk motion and magnetic field. By coupling the unique set of
observations available for the jet kinematics, environment and boundary profile
with our MHD modelling under assumption on the presence of a dynamically
important magnetic field in the M87 jet, we estimate the central black hole
mass and spin. The method leads us to believe that the M87 super massive black
hole has a mass somewhat larger than typically accepted so far.Comment: 10 pages, 1 figure, 3 tables, accepted for publication by MNRA
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